Object Oriented Programming SCJ2153. Class and Object. Associate Prof. Dr. Norazah Yusof

Transcription

1 Object Oriented Programming SCJ2153 Class and Object Associate Prof. Dr. Norazah Yusof

2 Classes Java program consists of classes. Class is a template for creating objects. Class normally consists of 3 components: 1. data members/attributes (also known as fields), 2. contructors (a special kind of method) 3. methods 2

3 Class Diagram Before a class is defined, it is always helpful to design using a class diagram. Class diagram is a set of standard diagram that graphically shows the object oriented system. The diagram consists of a box that is divided into 3 parts: Class name Attributes Constructors and methods 3

4 Structure of Class Diagram 4

5 Example of a Class Diagram Employee - empnum: int - empname: String + setempnum(int): void + getempnum(): int + setempname(string): void + getempname(): String 5

6 Class Diagram for Employee Employee - empnum: int - empname: String + setempnum(int): void + getempnum(): int + setempname(string): void + getempname(): String Class name Attributes / instance variables that define the object s state; can hold numbers, characters, strings, other objects Actions that an object of this class can take (behaviors) The symbol means private access modifier. The + symbol means public access modifier. 6

7 Structure of a Class Definition public class ClassName { // attributes definitions //constructor definitions public ClassName(paramList) { //method definitions public returntype methodname(paramlist) { 7

8 Class Definition public class Employee { private int empnum; private String empname; Employee - empnum: int - empname: String + setempnum(int): void + getempnum(): int + setempname(string): void + getempname(): String class diagram public void setempnum(int num) { empnum = num; public int getempnum() { return empnum; public void setempname(string name) { empname = name; public String getempname() { return empname; 8

9 Information Hiding Principle Accessor and Mutator method Information hiding using encapsulation Attributes are usually private Client application accesses them only through public interfaces Mutator method Controls data values used to set variable Accessor method Controls how value retrieved 9 9

10 Accessors and Mutators For the Employee class example, the accessors and mutators are: setempnum : Sets the value of the employee s number field. public void setempnum(int num) { setempname : Sets the value of the employee s name field. public void setempname(string name) getempnum : Returns the value of the employee s number field. public int getempnum() { getempname : Returns the value of the employee s name field. public String getempname() { Other names for these methods are getters and setters. 10

11 Declaring Object Reference Variable and Create Object To reference an object, assign the object to a reference variable. To declare a reference variable, use the syntax: ClassName objectrefvar; To create object: objectrefvar = new ClassName(); Example: Employee emp1; emp1 = Employee(); 11

12 Declaring/Creating Objects in a Single Step ClassName objectrefvar = new ClassName(); Assign object reference Create an object Example: Employee emp1 = new Employee(); 12

13 Accessing Objects After object instantiated, methods accessed using: Object s identifier Dot Data/Method call Referencing the object s data: objectrefvar.data Example: emp1.empnum Invoking the object s method: objectrefvar.methodname(arguments) Example: emp1.getempnum() 13

14 Constructors Special kind of methods for creating objects of a class, which play the role of initializing objects. The name of constructor must be the same as name of class. Constructors do not have a return type and they do not return a value. If a programmer does not define any constructors in a class, Java provides one default constructor i.e. a no argument constructor. 14

15 Constructors Constructors may take parameters. If a constructor has one parameter, it is called a one-argument constructor. If a class has more than one constructors, they must have different numbers and/or types of parameters. This is called constructor overloading Syntax: public ClassName(paramList) { 15

16 Class Definition with constructor public class Employee { private int empnum; private String empname; Employee - empnum: int - empname: String + Employee() + Employee (int, String) + getempnum(): int + getempname(): String class diagram public Employee() { System.out.println ( Start Employee ); public Employee(int num, String name) { empnum = num; empname = name; public int getempnum() { return empnum; public String getempname() { return empname; 16

17 Creating Object == invoking Constructors Constructors are invoked using the new operator when an object is created. Constructors play the role of initializing objects. Syntax: new ClassName(); Example: new Employee(); new Employee(123, Ahmad ); 17

18 Programming Example 18

19 Given the following class diagram about Travel write complete Java program for the traveller. 19

20 Travel.java public class Travel { private String destination, name, address; private double price; public Travel(){ public Travel(String name, String destination){ this.name = name ; this.destination = destination; public String getdestination(){ return destination; 20

21 Travel.java (cont.) public String getname(){ return name; public String getaddress(){ return address; public double getprice(){ return price; public void setdestination(string d){ destination = d; public void setname(string n){ name = n; 21

22 Travel.java (cont.) public void setaddress(string a){ address = a; public void setprice(double p){ price = p; public void display(){ System.out.println("\nDestination = "+destination); System.out.println("Name of traveller = "+name); System.out.println("Address of traveller = " + address); System.out.printf("Price = RM %.2f \n", price); 22

23 TestTravel.java public class TestTravel { public static void main(string[] args) { Travel ob1 = new Travel(); Travel ob2 = new Travel("Eyan","Indonesia"); ob1.setname("toh"); ob1.setdestination("italy"); ob1.setaddress("no.1 Tmn Emas,30220 Ipoh, Perak,Malaysia"); ob1.setprice(5999); ob1.display(); ob2.setaddress("n0.100 Tmn Raja,32223 Taiping, Malaysia"); ob2.setprice(3999); ob2.display(); 23